With reference to FIGS. 1, 2 and 3, a typical method of growing field oxide (SiO.sub.2) in a semiconductor device is shown. Initially (FIG. 2), a semiconductor substrate 10 is provided, and a thin layer of silicon dioxide (SiO.sub.2) 12 is grown thereover. A silicon nitride (Si.sub.3 N.sub.4) layer 14 is patterned over the oxide layer 12 as is well known, resulting in nitride layer portion 14A, 14B and 14C shown in cross section in FIG. 2. These nitride layer portions 14A, 14B, 14C may be quite long as shown in FIG. 1, and are used to define regions therebetween in which field oxide will be grown.
The thin oxide layer 12 is then patterned to the configuration of the nitride layer 14 (FIG. 2) forming oxide layer portions 12A, 12B, 12C. This leaves exposed areas 10A of the silicon substrate 10 which will subsequently be oxidized to form field silicon dioxide (SiO.sub.2) regions 16A, 16B, 16C.
An important fact for consideration is that during the conversion of silicon to silicon dioxide (FIG. 3), the volume of material from original silicon to silicon dioxide is increased by a factor of approximately two. With such field oxide typically being grown at 1100.degree. C., the silicon dioxide has a glass transition temperature at approximately 950.degree. C., so that at 950.degree. C. or greater, the silicon dioxide has a viscous flow. However, even with such viscous flow, the increase in volume as the silicon is converted to silicon dioxide, along with the fact that the silicon dioxide grows rapidly at 1100.degree. C., causes substantial stress to be placed on the edges of the nitride layer portions 14A, 14B, 14C as the oxide grows thereunder. That is, a large lifting force is applied to the nitride layer portions 14A, 14B, 14C along the elongated sides thereof, and particularly at the tip thereof where such lifting force is applied on three sides of the nitride layer portion 14A (FIG. 1). With dimensions of semiconductor devices becoming ever smaller, a width of a nitride layer portion 14A may for example be 0.5 micron. Because a small area of nitride layer portion 14A is in contact with the underlying oxide 12, which contact area grows smaller and smaller as the field oxide is grown inward under the nitride layer portion 14A, the force holding the nitride layer portion 14A in place may be reduced to the point where under the lifting force of the growing field oxide region, the nitride layer portion 14A lifts away from the thin oxide 12 to form a void 18 thereunder. And, as pointed out above, the faster the field oxide is grown, the less time is allowed for oxide flow to alleviate such stress on the nitride layer portion 14A.
Obviously, creation of such a void in a semiconductor device is undesirable and can lead to device failure.
In addition, with the contact area of the nitride layer portion 14A becoming smaller and smaller as the field oxide is grown inward under the nitride layer portion 14A, the silicon surface remaining for incorporation of circuit elements is reduced.